Radiation Applicator and Method of Radiating Tissue

ABSTRACT

A dipole microwave applicator emits microwave radiation into tissue to be treated. The applicator is formed from a thin coax cable having an inner conductor surrounded by an insulator, which is surrounded by an outer conductor. A portion of the inner conductor extends beyond the insulator and the outer conductor. A ferrule at the end of the outer conductor has a step and a sleeve that surrounds a portion of the extended inner conductor. A tuning washer is attached to the end of the extended inner conductor. A dielectric tip encloses the tuning washer, the extended inner conductor, and the sleeve of the ferrule. The sleeve of the ferrule and the extended inner conductor operate as the two arms of the dipole microwave antenna. The tuning washer faces the step in the ferrule, and is sized and shaped to cooperate with the step in balancing and tuning the applicator.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to commonly owned copendingInternational patent application no. WO 2006/002943, concerning aRadiation Applicator and Method of Radiating Tissue and which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to medical technology, and morespecifically to microwave radiation applicators and methods of thermalablative treatment of tissue using radiated microwaves.

2. Background Information

Thermal ablative therapies may be defined as techniques thatintentionally decrease body tissue temperature (hypothermia) orintentionally increase body tissue temperature (hyperthermia) totemperatures required for cytotoxic effect, or to other therapeutictemperatures depending on the particular treatment. Microwave thermalablation relies on the fact that microwaves form part of theelectromagnetic spectrum causing heating due to the interaction betweenwater molecules and the microwave radiation. The heat being used as thecytotoxic mechanism. Treatment typically involves the introduction of anapplicator into tissue, such as tumors. Microwaves are released from theapplicator forming a field around its tip. Heating of the watermolecules occurs in the radiated microwave field produced around theapplicator, rather than by conduction from the probe itself. Heating istherefore not reliant on conduction through tissues, and cytotoxictemperature levels are reached rapidly.

Microwave thermal ablative techniques are useful in the treatment oftumors of the liver, brain, lung, bones, etc.

U.S. Pat. No. 4,494,539 discloses a surgical operation method usingmicrowaves, characterized in that microwaves are radiated to tissue froma monopole type electrode attached to the tip of a coaxial cable fortransmitting microwaves. Coagulation, hemostasis or transaction is thenperformed on the tissue through the use of the thermal energy generatedfrom the reaction of the microwaves on the tissue. In this way, thetissue can be operated in an easy, safe and bloodless manner. Therefore,the method can be utilized for an operation on a parenchymatous organhaving a great blood content or for coagulation or transaction on aparenchymatous tumor. According to the method, there can be performed anoperation on liver cancer, which has been conventionally regarded asvery difficult. A microwave radiation applicator is also disclosed.

U.S. Pat. No. 6,325,796 discloses a microwave ablation assembly andmethod, including a relatively thin, elongated probe having a proximalaccess end, and an opposite distal penetration end adapted to penetrateinto tissue. The probe defines an insert passage extending therethroughfrom the access end to the penetration end thereof. An ablation catheterincludes a coaxial transmission line with an antenna device coupled to adistal end of the transmission line for generating an electric fieldsufficiently strong enough to cause tissue ablation. The coaxialtransmission line includes an inner conductor and an outer conductorseparated by a dielectric material. A proximal end of the transmissionline is coupled to a microwave energy source. The antenna device and thetransmission line each have a transverse cross-sectional dimensionadapted for sliding receipt through the insert passage while theelongated probe is positioned in the tissue. Such sliding advancementcontinues until the antenna device is moved to a position beyond thepenetration end and further into direct contact with the tissue.

However, a drawback with the existing techniques include the fact thatthey are not optimally mechanically configured for insertion into, andperforation of, the human skin, for delivery to a zone of soft tissue tobe treated. Typically, known radiation applicator systems do not havethe heightened physical rigidity that is desirable when employing suchtechniques.

In addition, some radiation applicators made available heretofore do nothave radiation emitting elements for creating a microwave field patternoptimized for the treatment of soft tissue tumors.

Also, given the power levels employed in some applicators andtreatments, there can be problems of unwanted burning of non-target,healthy tissue due to the very high temperatures reached by theapplicator or the components attached thereto.

Further, although small diameter applicators are known, and liquidcooling techniques have been used, there has been difficulty indesigning a small diameter device with sufficient cooling inapplications employing power levels required to deal with soft tissuetumors.

Accordingly, there is a need for methods of treatment of soft tissuetumors, and for radiation applicators that overcome any or all of theaforementioned problems of the prior art techniques, and provideimproved efficacy.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a dipole microwave applicator for emitting microwave radiationinto tissue, the assembly comprising: an outer conductor having an end;an inner conductor disposed within the outer conductor, and including asection that extends outwardly beyond the end of the outer conductor; aferrule disposed at the end of the outer conductor, and having a sleeveportion that surrounds a portion of the outwardly extending section ofthe inner conductor; and a dielectric tip surrounding the sleeve portionof the ferrule and the outwardly extending section of the innerconductor, whereby the sleeve portion of the ferrule and at least aportion of the outwardly extending section of the inner conductoroperate as corresponding arms of the dipole microwave applicator.

Particular embodiments are set out in the dependent claims.

Briefly, the present invention is directed to a microwave applicator forablating tissue. The applicator is a dipole microwave antenna thattransmits microwave radiation into the tissue being treated. Theapplicator is formed from a thin coaxial cable having an inner conductorsurrounded by an insulator, which is surrounded by an outer conductor orshield. The end of the coaxial cable is trimmed so that a portion of theinsulator and inner conductor extend beyond the outer conductor, and aportion of the inner conductor extends beyond the insulator. Theapplicator further includes a tubular ferrule defining an aperturetherethrough. One end of the ferrule is attached to the outer conductor,while the other end, which forms a sleeve, extends out beyond the end ofthe insulator and around a portion of the extended inner conductor. Astep is preferably formed on the outer surface of the ferrule betweenits two ends. A solid spacer having a central bore to receive the innerconductor abuts an end of the ferrule and surrounds the extended innerconductor. A tuning element is attached to the end of the extended innerconductor, and abuts an end of the spacer opposite the ferrule. Thetuning element faces the step in the ferrule, and the step and thetuning element are both sized and shaped to cooperate in balancing andtuning the applicator. A hollow tip, formed from a dielectric material,has an open end and a closed end. The tip encloses the tuning element,the spacer, and the extended inner conductor. The tip also encloses thesleeve of the ferrule, thus defining outer surface of the ferrule thatis surrounded by the dielectric tip. The open end of the tip preferablyabuts the step in the ferrule. A rigid sleeve surrounds the coaxialcable and extends away from the ferrule opposite the tip. The sleeve,which abuts the step of the ferrule opposite the tip, has an innerdiameter that is larger than the coaxial cable, thereby defining anannular space between the outside of the coaxial cable and the innersurface of the sleeve. The sleeve further includes one or more drainageholes, which permit fluid communication between the annular space aroundthe coaxial cable and the outside of the applicator.

In operation, microwave energy from a source is applied to the coaxialcable, and is conveyed to the tip. The portion of the inner conductorthat extends beyond the end of the ferrule forms one arm of the dipole,and emits microwave radiation. In addition, the microwave energy flowingalong the inner conductor of the coaxial cable and in the aperture ofthe ferrule induces a current to flow along the outer surface of thesleeve of the ferrule that is surrounded by the tip. This, in turn,causes microwave radiation to be emitted from the sleeve of the ferrule,which operates as the second arm of the dipole. In this way, microwaveenergy is emitted along a substantial length of the applicator, ratherthan being focused solely from the tip. By distributing the emission ofmicrowave radiation along a length of the applicator, higher powerlevels may be employed.

To keep the coaxial cable and the applicator from overheating, a coolingfluid is introduced from a source into the annular space defined by theoutside of the coaxial cable and the inside of the sleeve. The coolingfluid flows along this annular space, and absorbs heat from the coaxialcable. The cooling fluid, after having absorbed heat from the coaxialcable, then exits the annular space through the one or more drainageholes in the sleeve, and perfuses adjacent tissue.

The closed end of the tip is preferably formed into a blade or point sothat the Microwave applicator may be inserted directly into the tissuebeing treated. The tip, ferrule, and rigid sleeve, moreover, providestrength and stiffness to the applicator, thereby facilitating itsinsertion into tissue.

The present invention further provides a method of treating targettissue, such as a tumor, the tumor being formed of, and/or beingembedded within, soft tissue. The method includes inserting themicrowave applicator into the tumor, and supplying electromagneticenergy to the applicator, thereby radiating electromagnetic energy intothe tumor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic, partial cross-sectional view of a radiationapplicator in accordance with one embodiment of the invention;

FIG. 2A shows an axial cross-section, and FIG. 2B shows an end elevationof the radiating tip of the radiation applicator of FIG. 1;

FIG. 3 shows a partial transverse cross-section of the tube of theradiation applicator of FIG. 1;

FIG. 4A shows a transverse cross-section, and FIG. 4B shows an axialcross-section of the tuning washer of the radiation applicator of FIG.1;

FIG. 5A shows an axial cross-section, and FIG. 5B shows an end elevationof the ferrule of the radiation applicator of FIG. 1;

FIG. 6A shows an axial cross-section, and FIG. 6B shows a transversecross-section of a handle section that may be attached to the radiationapplicator of FIG. 1;

FIG. 7 illustrates the portion of coaxial cable that passes through thetube of the radiation applicator of FIG. 1;

FIG. 8 is a plot of S₁₁ against frequency for the radiation applicatorof FIG. 1;

FIG. 9A illustrates the E-field distribution, and FIG. 9B illustratesthe SAR values around the radiation applicator of FIG. 1, in use;

FIGS. 10A-E show a preferred sequential assembly of the radiationapplicator of FIG. 1;

FIG. 11 schematically illustrates a treatment system employing theradiation applicator of FIG. 1;

FIG. 12 is an exploded, perspective view of another embodiment of thepresent invention;

FIGS. 13-18 show a preferred sequential assembly of the radiationapplicator of FIG. 12; and

FIG. 19 is a schematic, partial cross-sectional view of the radiationapplicator of FIG. 12.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

In the following description, like references are used to denote likeelements, and where dimensions are given, they are in millimeters (mm).Further, it will be appreciated by persons skilled in the art that theelectronic systems employed, in accordance with the present invention,to generate, deliver and control the application of radiation to partsof the human body may be as described in the art heretofore. Inparticular, such systems as are described in commonly owned publishedinternational patent applications W095/04385, W099/56642 and WOOO/49957may be employed (except with the modifications described hereinafter).Full details of these systems have been omitted from the following forthe sake of brevity.

FIG. 1 is a schematic, partial cross-sectional view of a radiationapplicator in accordance with one embodiment of the invention. Theradiation applicator, generally designated 102, includes a distal endportion of a coaxial cable 104 that is used to couple to a source (notshown) of microwaves, a copper ferrule 106, a tuning washer 108 attachedon the end 110 of the insulator part of the coaxial cable 104, and a tip112. Preferably, the applicator 102 further includes a metal tube 114.Tube 114 is rigidly attached to the ferrule 106. An annular space 116 isdefined between the outer conductor 118 of the cable 104 and the innersurface of the tube 114, enabling cooling fluid to enter (in thedirection of arrows A), contact the heated parts of the applicator 102and exit in the direction of arrows B through radial holes 120 in thetube 114, thereby extracting heat energy from the radiation applicator102.

In assembly of the applicator 102, the washer 108 is soldered to a smalllength 122 of the central conductor 124 of the cable 104 that extendsbeyond the end 110 of the insulator 126 of the cable 104. The ferrule106 is soldered to a small cylindrical section 128 of the outerconductor 118 of the cable 104. Then, the tube 114, which is preferablystainless steel, but may be made of other suitable materials, such astitanium or any other medical grade material, is glued to the ferrule106 by means of an adhesive, such as Loctite 638 retaining compound, atthe contacting surfaces thereof, indicated at 130 and 132. The tip 112is also glued preferably, using the same adhesive, on the inner surfacesthereof, to corresponding outer surfaces of the ferrule 106 and theinsulation 126.

When assembled, the applicator 102 forms a unitary device that is rigidand stable along its length, which may be of the order of 250 or somillimeters including tube 114, thereby making the applicator 102suitable for insertion into various types of soft tissue. The space 116and holes 120 enable cooling fluid to extract heat from the applicator102 through contact with the ferrule 106, the outer conductor 118 of thecable 104 and the end of the tube 114. The ferrule 106 assists, amongother things, in assuring the applicator's rigidity. The exposed endsection 134 of cable 104 from which the outer conductor 118 has beenremoved, in conjunction with the dielectric tip 112, are fed by a sourceof radiation of predetermined frequency. The exposed end section 134 anddielectric tip 112 operate as a radiating antenna for radiatingmicrowaves into tissue for therapeutic treatment. The applicator 102operates as a dipole antenna, rather than a monopole device, resultingin an emitted radiation pattern that is highly beneficial for thetreatment of certain tissues, such as malignant or tumorous tissue, dueto its distributed, spherical directly heated area.

FIG. 2A shows an axial cross-section, and FIG. 2B shows an end elevationof the tip 112 of the radiation applicator 102 of FIG. 1. As can beseen, the tip 112 has inner cylindrical walls 202, 204, and abuttingwalls 206, 208, for receiving and abutting the washer 108 and theferrule 106, respectively, during assembly. Suitably, the tip 112 ismade of zirconia ceramic alloy. More preferably, it is a partiallystabilized zirconia (PSZ) having yttria as the stabilizing oxidizingagent. Even more preferably, the tip 112 is made of Technox 2000, whichis a PSZ commercially available from Dynamic Cerarnic Ltd. ofStaffordshire, England, having a very fine uniform grain compared toother PSZs, and a dielectric constant (k) of 25. As understood by thoseskilled in the art, the choice of dielectric material plays a part indetermining the properties of the radiated microwave energy.

It will be noted that the transverse dimensions of the applicator 102are relatively small. In particular, the diameter of applicator 102 ispreferably less than or equal to about 2.4 mm. The tip 112, moreover, isdesigned to have dimensions, and be formed of the specified material, soas to perform effective tissue ablation at the operating microwavefrequency, which in this case is preferably 2.45 Gigahertz (GHz). Theapplicator 102 of the present invention is thus well adapted forinsertion into, and treatment of, cancerous and/or non-cancerous tissueof the liver, brain, lung, veins, bone, etc.

The end 210 of the tip 112 is formed by conventional grinding techniquesperformed in the manufacture of the tip 112. The end 210 may be formedas a fine point, such as a needle or pin, or it may be formed with anend blade, like a chisel, i.e. having a transverse dimension ofelongation. The latter configuration has the benefit of being wellsuited to forcing the tip 112 into or through tissue, i.e. to perforateor puncture the surface of tissue, such as skin.

In use, the tip 112 is preferably coated with a non-stick layer such assilicone or paralene, to facilitate movement of the tip 112 relative totissue.

FIG. 3 shows a partial transverse cross-section of the tube 114. Asmentioned above, the tube 114 is preferably made of stainless steel.Specifically, the tube 114 is preferably made from 13 gauge thin wall304 welded hard drawn (WHD) stainless steel. The tube 114 is alsoapproximately 215 mm in length. As can be seen, two sets of radial holes120, 120′ are provided at 12 mm and 13 mm, respectively, from the end302 of the tube 114. These radial holes 120, 120′, as mentioned, permitthe exit of cooling fluid. Although two sets of holes are shown, one,three, four or more sets of holes may be provided, in variants of theillustrated embodiment. In addition, although two holes per set areshown, three, four, five, or more holes per set may be provided, so longas the structural rigidity of the tube 114 is not compromised. In thisembodiment, the holes 120, 120′ are of 0.5 mm diameter, but it will beappreciated that this diameter may be quite different, e.g. any thing inthe range of approximately 0.1 to 0.6 mm, depending on the number ofsets of holes and/or the number of holes per set, in order to provide aneffective flow rate. Although the illustrated distance from the end 302is 12 or 13 mm, in alternative embodiments, this distance may range from3 mm to 50 mm from the end 302, in order to control the length of trackthat requires cauterization.

Further, in an embodiment used in a different manner, the tube 114 maybe omitted. In this case the treatment may comprise delivering theapplicator to the treatment location, e.g., to the tumorous tissue, bysuitable surgical or other techniques. For example, in the case of abrain tumor, the applicator may be left in place inside the tumor, theaccess wound closed, and a sterile connector left at the skull surfacefor subsequent connection to the microwave source for follow-uptreatment at a later date.

FIG. 4A shows a transverse cross-section, and FIG. 4B shows an axialcross-section of the tuning washer 108. The washer 108 is preferablymade of copper, although other metals may be used. The washer 108 has aninner cylindrical surface 402 enabling it to be soldered to the centralconductor 124 of the cable 104 (FIG. 1). Although the washer is small,its dimensions are critical. The washer 108 tunes the applicator 102,which operates as a dipole radiator, i.e., radiating energy from twolocations, so that more effective treatment, i.e., ablation, of tissueis effected.

FIG. 5A shows an axial cross-section, and FIG. 5B shows an end elevationof the ferrule 106. The ferrule 106 is preferably made of copper, and ispreferably gold plated to protect against any corrosive effects of thecooling fluid. The ferrule 106 may be produced by conventional machiningtechniques, such as CNC machining.

FIG. 6A shows an axial cross-section, and FIG. 6B shows a transversecross-section at line B-B of a handle section 602 that may be attachedto the tube 114 of the radiation applicator 102. The handle section 602is preferably made from the same material as the tube 114, i.e.,stainless steel. The handle section 602 includes a forward channel 604enabling insertion of the tube 114, and a rear channel 606 enablinginsertion of the coaxial cable 104 during assembly. A transverse port608 having an internal thread 610 enables the connection, through aconnector, to a source of cooling fluid, discussed later. The connectormay be formed from plastic. Once assembled, the arrangement of handlesection 602 enables cooling fluid to pass in the direction of arrow Cinto the tube 114 (not shown).

FIG. 7 illustrates the portion of coaxial cable 104 that passes throughthe tube 114. The cable 104 suitably comprises a low-loss, coaxial cablesuch as SJS070LL-253-Strip cable. A connector 702, preferably a SMAfemale type connector permits connection of the cable 104 to a microwavesource (not shown), or to an intermediate section of coaxial cable (notshown) that, in turn, connects to the microwave source.

FIG. 8 is a plot of S11 against frequency for the radiation applicator102 of FIG. 1. This illustrates the ratio of reflective microwave powerfrom the interface of the applicator 102 and treated tissue to totalinput power to the applicator 102. As can be seen, the design of theapplicator 102 causes the reflected power to be a minimum, and thereforethe transmitted power into the tissue to be a maximum, at a frequency of2.45 GHz of the delivered microwaves.

FIG. 9A shows the E-field distribution around the radiation applicator102 of FIG. 1, in use. Darker colors adjacent to the applicator 102indicate points of higher electric field. In FIG. 9A, the position ofthe washer 108 is indicated at 902, and the position of the tip-ferrulejunction is indicated at 904. Two limited, substantially cylindricalzones 906, 908, of highest electric field are formed around theapplicator 102 at the positions 902 and 904 respectively.

FIG. 9B shows the specific absorption rate (SAR) value distributionaround the radiation applicator 102 of FIG. 1, in use. Darker colorsadjacent the applicator 102 indicate points of SAR. In FIG. 9B, theposition of the washer 108 is indicated at 902, the position of thetip-ferrule junction is indicated at 904, and the position of theferrule-tube junction is indicated at 905. Two limited, substantiallycylindrical zones 910, 912, of highest SAR are formed around theapplicator 102 at the positions 902 and between 904 and 905,respectively.

FIGS. 10A-E show a preferred sequential assembly of components formingthe radiation applicator 102 of FIG. 1. In FIG. 10A, the coaxial cable104 is shown with the outer conductor 118 and the inner insulator 126trimmed back, as illustrated earlier in FIG. 7.

As shown in FIG. 10B, the tube 114 is then slid over the cable 104.Next, the ferrule 106 is slid over the cable 104 (FIG. 10C), and fixedlyattached to the tube 114 and to the cable 104, as described earlier.Then, the washer 108 is attached to the inner conductor 124 bysoldering, as shown in FIG. 1D. Finally, the tip 112 is slid over thecable 104 and part of the ferrule 106, and affixed thereto, as describedearlier. The completed applicator is shown in FIG. 10E. This results ina construction of great rigidity and mechanical stability.

FIG. 11 schematically illustrates a treatment system 1102 employing theradiation applicator 102 of FIG. 1. Microwave source 1104 is couple tothe input connector 1106 on handle 602 by coaxial cable 1108. In thisembodiment, the microwave power is supplied at up to 80 Watts. Howeverthis could be larger for larger size applicators, e.g., up to 200 Wattsfor 5 mm diameter radiation applicators.

Syringe pump 1110 operates a syringe 1112 for supplying cooling fluid1114 via conduit 1116 and connector 1118 attached to handle 602, to theinterior of the handle section 602. The fluid is not at great pressure,but is pumped so as to provide a flow rate of about 1.5 to 2.0milliliter(ml)/minute through the pipe 114 in the illustratedembodiment. However, in other embodiments, where the radiationapplicator 102 is operated at higher powers, higher flow rates may beemployed, so as to provide appropriate cooling. The cooling fluid ispreferably saline, although other liquids or gases may be used, such asethanol. In certain embodiments, a cooling liquid having a secondary,e.g., cytotoxic, effect could be used, enhancing the tumor treatment. Inthe illustrative embodiment, the cooling fluid 1114 exits the tube 1114,as shown by arrows B in FIG. 1, at a temperature on the order of 10° C.higher than that at which it enters the tube 114, as shown by arrows Ain FIG. 1. Thus, substantial thermal energy is extracted from thecoaxial cable. The cooling fluid 1114 may, for example, enter the tube114 at room temperature. Alternatively, the cooling fluid 1114 may bepre-cooled to a temperature below room temperature by any suitabletechnique.

As shown, the cooling system is an open, perfusing cooling system thatcools the coaxial cable connected to the radiation applicator 102. Thatis, after absorbing heat from the coaxial cable, the cooling fluidperfuses the tissue near the radiation applicator 102.

The methodology for use of the radiation applicator 102 of the presentinvention may be as conventionally employed in the treatment of varioussoft tissue tumors. In particular, the applicator 102 is inserted intothe body, laparoscopically, percutaneously or surgically. It is thenmoved to the correct position by the user, assisted where necessary bypositioning sensors and/or imaging tools, such as ultrasound, so thatthe tip 112 is embedded in the tissue to be treated. The microwave poweris switched on, and the tissue is thus ablated for a predeterminedperiod of time under the control of the user. In most cases, theapplicator 102 is stationary during treatment. However, in someinstances, e.g., in the treatment veins, the applicator 102 may bemoved, such as a gentle sliding motion relative to the target tissue,while the microwave radiation is being applied.

As described above, and as shown in FIGS. 9A and 9B, radiationapplicator 102, is a dipole antenna. The portion of the inner conductor124 that extends beyond the ferrule 106 operates as one arm of thedipole antenna. In addition, the transmission of microwave energy alongthe inner conductor 124 and in the aperture of the ferrule induces acurrent to flow on that portion of the outer surface of the ferrule 106that is located underneath the tip 112. This induced current causes thisenclosed, outer surface of the ferrule 106 to emit microwave radiation,thereby forming a second arm of the dipole antenna. The bipolarconfiguration of the applicator effectively spreads the microwaveradiation that is being transmitted by the applicator 102 along agreater transverse, i.e., axial, length of the antenna 102, rather thanfocusing the radiation transmission solely from the tip 112 of theapplicator 102. As a result, the applicator 102 of the present inventionmay be operated at much higher power levels, e.g., up to approximately80 Watts, than prior art designs.

An alternative embodiment of the present invention is shown in FIGS.12-19. FIG. 12 is an exploded, perspective view of an alternativeradiation applicator 1202. As shown, the applicator 1202 includes acoaxial cable 1204 having an outer conductor 1206 that surrounds aninsulator 1208 that, in turn, surrounds an inner or central conductor1210. The applicator 1202 further includes a ferrule 1212. The ferrule1212 is generally tubular shaped so as to define an aperturetherethrough, and has first and second ends 1212 a, 1212 b. The ferrule1212 also has three parts or sections. A first section 1214 of theferrule 1212 has an inner diameter sized to fit over the outer conductor1206 of the coaxial cable 1204. A second section 1216 of the ferrule1212 has an inner diameter that is sized to fit over the insulator 1208of the coaxial cable 1204. The second section 1216 thus defines anannular surface or flange (not shown) around the inside the ferrule1212. The outer diameter of the second section 1216 is preferably largerthan the outer diameter of the first section 1214, thereby defining astep or flange around the outside of the ferrule 1212. A third section1218 of the ferrule 1212 has an inner diameter also sized to fit aroundthe insulator 1208 of the coaxial cable 1204. The third section 1218 hasan outside diameter that is less than the outside diameter of the secondsection 1216. The third section 1218 thus defines an outer, cylindricalsurface or sleeve.

Applicator 1202 further includes a spacer 1220. The spacer 1220 ispreferably cylindrical in shape with a central bore 1222 sized toreceive the inner conductor 1210 of the coaxial cable 1204. The outerdiameter of the spacer 1220 preferably matches the outer diameter of thethird section 1218 of the ferrule 1212. Applicator 1202 also includes atuning element 1224 and a tip 1226. The tuning element 1224, which bemay be disk-shaped, has a central hole 1228 sized to fit around theinner conductor 1210 of the coaxial cable 1204. The tip 1226 is ahollow, elongated member, having an open end 1230, and a closed end1232. The closed end 1232 may be formed into a cutting element, such asa trocar point or a blade, to cut or pierce tissue. Applicator 1202 alsoincludes a rigid sleeve 1234. The sleeve 1234 has an inner diameter thatis slightly larger than outer diameter of the coaxial cable 1204. Asdescribed below, an annular space is thereby defined between the outersurface of the coaxial cable 1204 and the inner surface of the sleeve1234. The sleeve 1234 further includes one or more drainage holes 1236that extend through the sleeve.

FIGS. 13-18 illustrate a preferred assembly sequence of the applicator1202. As shown in FIG. 13, the coaxial cable 1204 is trimmed so thatthere is a length “m” of insulator 1208 that extends beyond an end 1206a of the outer conductor 1206, and a length “l” of inner conductor 1210that extends beyond an end 1208 a of the insulator 1208. The ferrule1212 slides over the exposed inner conductor 1210 and over the exposedinsulator 1208 such that the first section 1214 surrounds the outerconductor 1206, and the second and third sections 1216, 1218 surroundthe exposed portion of the insulator 1208. The inner surface or flangeformed on the second section 1216 of the ferrule 1212 abuts the end 1206a of the outer conductor 1206, thereby stopping the ferrule 1212 fromsliding any further up the coaxial cable 1204. The ferrule 1212 ispreferably fixedly attached to the coaxial cable 1204, such as bysoldering the ferrule 1212 to the outer conductor 1206 of the coaxialcable 1204. In the preferred embodiment, the third section 1218 of theferrule 1212 extends past the end 1208 a of the exposed insulator 1208as shown by the dashed line in FIG. 14.

Next, the spacer 1220 is slid over the exposed portion of the innerconductor 1210, and is brought into contact with the second end 1212 bof the ferrule 1212. In the preferred embodiment, the spacer 1220 is notfixedly attached to the ferrule 1212 or the inner conductor 1210. Thespacer 1220 is sized so that a small portion 1210 a (FIG. 15) of theinner conductor 1210 remains exposed. The tuning element 1224 is thenslid over this remaining exposed portion 1210 a of the inner conductor1210. The tuning element 1224 is preferably fixedly attached to theinner conductor 1210, e.g., by soldering. The tuning element 1224, incooperation with the ferrule 1212, thus hold the spacer 1220 in place.

With the tuning element 1224 in place, the next step is to install thetip 1226 as shown in FIG. 16. The open end 1230 of the tip 1226 is slidover the tuning element 1224, the spacer 1224 and the third section 1218of the ferrule 1212. The open end 1230 of the tip 1226 abuts the secondsection or step 1216 of the ferrule 1212. The tip 1226 is preferablyfixedly attached to the ferrule 1212, e.g., by bonding. With the tip1226 in place, the next step is to install the sleeve 1234 (FIG. 17).The sleeve 1234 is slid over the coaxial cable 1234, and up over thefirst section 1214 of the ferrule 1212. The sleeve 1234 abuts the step1216 in the ferrule 1212 opposite the tip 1226.

Those skilled in the art will understand that the applicator 1202 may beassembled in different ways or in different orders.

As illustrated in FIG. 18, upon assembly, the tip 1226, second section1216 of the ferrule 1212, and sleeve 1234 all preferably have the sameouter diameter, thereby giving the applicator 1202 a smooth outersurface.

Preferably, the sleeve 1234 is formed from stainless steel, and theferrule 1212 is formed from gold-plated copper. The tip 1226 and thespacer 1220 are formed from dielectric materials. In the illustrativeembodiment, the tip 1226 and the spacer 1220 are formed from an itriumstabilized zirconia, such as the Technox brand of ceramic materialcommercially available from Dynamic Ceramic Ltd. of Stoke-on-Trent,Staffordshire, England, which has a dielectric constant of 25. The tip1226 may be further provided with a composite coating, such as apolyimide undercoat layer, for adhesion, and a paralyne overcoat layer,for its non-stick properties. Alternatively, silicone or some othersuitable material could be used in place of paralyne. The compositecoating may also be applied to the ferrule and at least part of thestainless steel sleeve, in addition to being applied to the tip.

Those skilled in the art will understand that alternative materials maybe used in the construction of the radiation applicator 1202.

FIG. 19 is a schematic, partial cross-sectional view of the radiationapplicator 1202. As shown, at least part of the first section 1214 ofthe ferrule 1212 overlies and is attached to the outer conductor 1206.The insulator 1208 extends partially through the inside of the ferrule1212. In particular, the end 1208 a of the insulator 1208 is disposed apredetermined distance back from the second end 1212 b of the ferrule1212. The inner conductor 1210 extends completely through and beyond theferrule 1212. The sleeve 1234 slides over and is bonded to the firstsection 1214 of the ferrule 1212. As shown, the inside diameter of thesleeve 1234 is greater than the outside diameter of the coaxial cable1204, thereby defining an annular space 1238 between the outside of thecoaxial cable 1204 and the inside of the sleeve 1234. Cooling fluid,such as saline, is pumped through this annular space 1238, as shown byarrows A. The cooling fluid absorbs heat from the coaxial cable thatfeeds radiation to applicator 1202. The cooling fluid is then dischargedthrough holes 1236 in the sleeve 1234, as shown by arrows B.

In the preferred embodiment, the holes 1236 are placed far enough behindthe closed end 1232 of the tip 1226 such that the discharged coolingfluid does not enter that portion of the tissue that is being heated bythe radiation applicator 1202. Instead, the discharged cooling fluidpreferably perfuses tissue outside of this heated region. Depending onthe tissue to be treated, a suitable distance between the closed end1232 of the tip 1226 and the holes 1236 may be approximately 30 mm.

A first end 1220 a of the spacer 1220 abuts the second end 1212 b of theferrule 1212, while a second end 1220 b of the spacer 1220 abuts thetuning element 1224. Accordingly a space, designated generally 1240, isdefined within the ferrule 1212 between the end 1208 a of the insulatorand the second end 1212 b of the ferrule. In the illustrativeembodiment, this space 1240 is filled with air. Those skilled in the artwill understand that the space may be filled with other materials, suchas a solid dielectric, or it may be evacuated to form a vacuum. Theinside surface of the tip 1226 preferably conforms to the shape of thetuning element 1224, the spacer 1220, and the third section 1218 of theferrule 1212 so that there are no gaps formed along the inside surfaceof the tip 1226.

As indicated above, operation of the radiation applicator 1202 causes acurrent to be induced on the outer surface of the third section 1218 ofthe ferrule 1212, which is enclosed within the dielectric material ofthe tip 1226. This induced current results in microwave energy beingradiated from this surface of the ferrule 1212, thereby forming one armof the dipole. The section of the inner conductor 1210 that extendsbeyond the ferrule 1212 is the other arm of the dipole. Both the lengthof the inner conductor 1210 that extends beyond the ferrule 1212, andthe length of the third section 1218 of the ferrule 1212, which togethercorrespond to the two arms of the dipole, are chosen to be approximately¼ of the wavelength in the dielectric tip 1226, which in theillustrative embodiment is approximately 6 mm. Nonetheless, thoseskilled in the art will understand that other factors, such as tissuepermittivity, the action of the tuning element, etc., will affect theultimate lengths of the dipole arms. For example, in the illustrativeembodiment, the two arms are approximately 5 mm in length.

The tuning element 1224, moreover, cooperates with the second section orstep 1216 of the ferrule to balance the radiation being emitted by thetwo arms of the dipole. In particular, the size and shape of the tuningelement 1224 and the step 1216 are selected such that the coherent sumof the microwave power reflected back toward the cable at the apertureof the ferrule is minimized. Techniques for performing such designoptimizations are well-known to those skilled in the relevant art.

In use, the radiation applicator 1202 is attached to a source ofmicrowave radiation in a similar manner as described above in connectionwith the applicator 102 of FIG. 1. The coaxial cable is also attached toa source of cooling fluid in a similar manner as described above. Withthe present invention, it is the dielectric tip, ferrule and stainlesssteel sleeve that cooperate to provide the necessary stiffness andmechanical strength for the applicator to be used in treatmentprocedures. The applicator does not rely on the coaxial cable for any ofits strength. Indeed, a flexible coaxial cable, having little or norigidity, could be used with the radiation applicator of the presentinvention.

The foregoing has been a detailed description of illustrativeembodiments of the invention. Various modifications and additions can bemade without departing from the spirit and scope thereof. For example,the materials described herein are not exhaustive, and any acceptablematerial can be employed for any component of the described system andmethod. In addition, modifications can be made to the shape of variouscomponents. Accordingly, this description is meant to be taken only byway of example, and not to otherwise limit the scope of the invention.

1. A dipole microwave applicator for emitting microwave radiation intotissue, the assembly comprising: an outer conductor having an end; aninner conductor disposed within the outer conductor, and including asection that extends outwardly beyond the end of the outer conductor; aferrule disposed at the end of the outer conductor, and having a sleeveportion that surrounds a portion of the outwardly extending section ofthe inner conductor; and a dielectric tip surrounding the sleeve portionof the ferrule and the outwardly extending section of the innerconductor, whereby the sleeve portion of the ferrule and at least aportion of the outwardly extending section of the inner conductoroperate as corresponding arms of the dipole microwave applicator.
 2. Thedipole microwave applicator of claim 1, further comprising a dielectricspacer disposed within the dielectric tip, the dielectric spacersurrounding at least a portion of the inner conductor that extendsbeyond the sleeve portion of the ferrule.
 3. The dipole microwaveapplicator of claim 1, wherein the ferrule has a first end that isattached to the end of the outer conductor.
 4. The dipole microwaveapplicator of claim 1, further comprising a tuning element disposedwithin the dielectric tip, and attached to an end of the innerconductor.
 5. The dipole microwave applicator of claim 4, wherein theferrule further includes a step adjacent to the sleeve portion, and thetuning element and step cooperate to balance the corresponding arms ofthe dipole microwave applicator.
 6. The dipole microwave applicator ofclaim 5, wherein the tuning element is substantially disc shaped.
 7. Thedipole microwave applicator of claim 5, further comprising a rigidsleeve adjacent to the ferrule, and surrounding and spaced from at leasta portion of the outer conductor so as to define a space between theouter conductor and the rigid sleeve.
 8. The dipole microwave applicatorof claim 7, wherein one or more holes extend through the rigid sleeve,the one or more holes providing a fluid communication path from thespace within the rigid sleeve to an area outside of the rigid sleeve. 9.The dipole microwave applicator of claim 1, wherein the ferrule isformed from copper, and the tip is formed from itrium stabilizedzirconia.
 10. The dipole microwave applicator of claim 8, wherein thesleeve is formed from stainless steel, the ferrule is formed fromcopper, and the tip is formed from itrium stabilized zirconia.
 11. Thedipole microwave applicator of claim 1, wherein microwave energy at afrequency of approximately 2.45 Gigahertz (GHz) and a power level of upto 80 watts is applied to the applicator.
 12. The dipole microwaveapplicator of claim 2, further comprising an insulator disposed betweenthe outer conductor and the inner conductor, wherein the spacer abuts anend of the sleeve of the ferrule and the insulator terminates within thesleeve so as to define a gap within the sleeve of the ferrule around theinner conductor.
 13. The dipole microwave applicator of claim 10,wherein the gap is filled with air.
 14. The dipole microwave applicatorof claim 5, wherein the dielectric tip has open end that abuts the stepin the ferrule and a closed end opposite the open end, and the closedend is configured for one of cutting or piercing tissue.
 15. The dipolemicrowave applicator of claim 1, wherein at least one of the dielectrictip and the ferrule is coated with an inner layer of polyimide and anouter layer of paralyne.